1. Field of the Invention
The present invention relates to an X-ray transmission inspection apparatus and an X-ray transmission inspection method which are capable of detecting a foreign matter formed of a particular element in a sample.
2. Description of the Related Art
In recent years, a lithium-ion secondary battery, which has a higher energy density compared with that of a nickel-metal hydride battery, has been increasingly adopted as a battery for an automobile, a hybrid car, an electric vehicle, and the like. The lithium-ion secondary battery is a kind of a non-aqueous electrolyte secondary battery, which has lithium ions in an electrolyte conducting electricity and does not contain metal lithium, and has already been used widely in notebook personal computers and mobile phones.
The lithium-ion secondary battery has excellent battery characteristics. However, when a foreign matter such as Fe (iron) enters in an electrode during a production process, the reliability of the battery is adversely affected, such as degradation in battery characteristics as to a heat-generation property, a longevity, and the like, which has delayed installation of the battery on a vehicle. For example, an electrode (anode) of the lithium-ion secondary battery is generally formed in such a manner that a lithium manganese oxide film or a lithium cobalt oxide film is formed to have a thickness of about 100 μm on both surfaces of an Al film having a thickness of 20 μm. A foreign matter such as Fe (iron) or SUS (stainless) may be mixed into the electrode, and if the size of the foreign matter is tens of μm or larger, short-circuit occurs, and may cause burning of the battery and decrease in performance. Therefore, there is a demand in the lithium-ion secondary battery that a battery with a foreign matter X mixed therein during production be detected swiftly and the foreign matter X be removed in advance.
In general, as a method of detecting a foreign matter or the like in a sample, a method using a transmitted X-ray image is known. Using this procedure, for example, as disclosed in Japanese Unexamined Patent Application Publication No. 2001-91480 (in claims), a method of detecting a presence/absence of foreign matter through the transmitted X-ray image has been proposed conventionally.
The above-mentioned related art has the following problem.
That is, according to a conventional method of detecting a foreign matter, clearness of an image of a foreign matter varies between the case where the position of the foreign matter is close to an X-ray detector and the case where the foreign matter is far away from the X-ray detector. Therefore, according to the conventional method of detecting a foreign matter, a large difference in contrast caused by the foreign matter occurs between the case where the foreign matter is close to the X-ray detector as illustrated in FIG. 2 and the case where the foreign matter is far away from the X-ray detector as illustrated in FIG. 3, and an image to be taken varies even with a foreign matter of the same size and the same material. This leads to a problem such as overdetection or erroneous detection.